[openacc][openmp] Reduce number of generated file and prefer inclusion of .inc

Follow up from D92955 and D83636. This patch makes the base cpp files
OMP.cpp and ACC.cpp normal files and they now include the XXX.inc file
generated by tablegen. This reduces the number of file generated by the
DirectiveEmitter backend and makes it closer to the proposal in D83636.

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D93560